PROJECT SUMMARY - ABSTRACT The maintenance of salt and water balance is essential to the functional stability of the macromolecules that carry on the business of life. Failure to properly maintain osmotic equilibrium is a contributing factor to the pathologies of nearly every organ system. Changes in extracellular osmolality are detected by osmoreceptors, cells that are osmosensitive and capable of producing an effector, typically a hormone, that is capable of restoring osmotic equilibrium. Because little is known about the modulation of osmoreceptors, the characterization of osmotically and hormonally induced regulation of osmoregulatory effectors represents a promising tool to advance our fundamental knowledge on how to counteract osmoregulatory disequilibrium. This approach can be especially valuable if the model for characterization of such regulatory mechanisms is naturally and strongly responsive to subtle, physiologically appropriate, osmotic stimuli. By employing an accessible fish cell model that is remarkably sensitive and responsive to physiological changes in extracellular osmolality, we seek to identify novel mechanisms that regulate osmoreception and autocrine regulation. The proposed research comprises two main overlapping goals: 1) Characterize the effects of autocrine regulation of a potent osmoregulatory hormone, the tilapia prolactin (Prl), on Prl cells. Tilapia Prl cells are an established vertebrate endocrine model for osmoreception. A fall in extracellular osmolality stimulates its main secretory product, Prl, which exerts ion-retaining activity on target osmoregulatory tissues. Preliminary evidence indicates that the autocrine regulation of two isoforms of Prl are differentially modulated by extracellular osmolality; and 2) identify the mechanism(s) underlying changes in osmotically-modulated autocrine regulation. Two Prl receptors have been identified and shown to be differentially osmosensitive. Their involvement in the mediation of autocrine regulation by Prls will be assessed pharmacologically. Findings from the proposed studies shall provide fundamental knowledge on the regulation of osmoreceptors and endocrine cells, which may ultimately lead to the development of therapeutic intervention strategies for osmoregulatory dysfunction.